This study recruits preterm infants born less than 32 weeks gestational age and treated at the neonatal intensive care unit of Oulu University Hospital. The informed consent is asked from all parents. The first drug dose is given before 24 hours of age. Masked study drug is paracetamol infusion solution 10 mg/mL (PERFALGAN ®) or placebo, 0.45% saline solution. The loading dose is 20 mg/kg, and the maintenance dose 7.5 mg/kg every 6 hours for 4 days. The exact date of the closure of ductus is studied by repeated echocardiographic examinations. The symptoms of pain are screened by a locally used pain scale of preterm infants (NIAPAS). Patients are monitored for signs of possible side effects. After discharge from hospital, patients are examined at follow-up clinic for the first year every 3 months and at 2 years of age.